| As the concept of quantum computer was proposed by Richard Fynman in 1982,quantum computers and quantum algorithms have been attracting more and more attention in recent years for their excellent computing power.Nowadays,there are several leading quantum processing platforms in the world:superconducting circuit,NV centers,ion trap,cold atoms,linear optics and semiconductor quantum dots.Strained Germanium hole spin qubits are one of the most promising platforms for realizing large-scale quantum computing.Compared to the group III-V compounds like GaAs,in which spin qubits are heavily affected by coupling with nuclear spin,there is only little nuclear spin in group Ⅳ elements such as silicon and germanium.The impact of hyperfine interactions on spin decoherence time T2*can be further reduced by isotope purification.Germanium has the highest mobility among conventional semiconductor materials and the effective mass of holes is much smaller than that of electrons.Therefore,the size of germanium hole quantum dots can be much larger than that of silicon electron quantum dots,reducing the difficulty of micro-nano fabrication.Moreover,micronano structures like micromagnet and electron diople resonance line are needed to relize the control of electron spin qubits in silicon quantum dots.However,there is strong intrinsic spin orbital coupling in germanium quantum dots,which ensures fast all-electric field manipulation of spin qubits without additional micronano structures.This greatly simplifies the fabrication process of quantum dot devices,improving the yield of quantum devices and increasing the two-dimensional scalability of quantum dots.Moreover,germanium quanutm dots are well compatible with industry-standard semiconductor fabrication,showing the wonderful prospects in scalable quantum technology and large-scale fabrication.Due to the excellent properties of semiconductor germanium materials,this paper focuses on exploring the fabrication and properties of multi-quantum dot devices fabricated in strain germanium heterostructure,demonstrating the good tunability of quantum dots.I detailedly study the spin blockade in germanium and its lift mechanism,proving the system has strong spin-orbit coupling strength.Then I fabricate and characterize the one-dimensional quadruple quantum dot devices.These works have laid a solid foundation for the realization of high-fidelity spin qubits of germanium quantum dots.My work is mainly divided into the following parts:1.Starting from the history and development of quantum computing,I introduce the basic concepts of quantum computing such as quanutm dots and qubits,as well as the development frontiers of large-scale integrated quantum dot arrays by international and domestic research groups.Next,I exhibit the properties,manipulation method and readout method of hole spin qubits,as well as the characteristics of Pauli spin blockade and spin-orbit coupling.2.I introduce the structure and growth method of strain Germanium heterostructure substrate,as well as the micro-nano fabrication and measurement process of multiquantum dot devices.Meanwhile,I briefly list the properties and basic usage of the fabrication and measurement platforms.Finally I fabricate the double quantum dot devices and one-dimensional quadruple quantum dot devices with good tunability.3.I characterize the properties of the strained germanium heterostructure substrate along with the AC and DC transport properties of single and double quantum dots,realizing a wide range of coupling tunability.I study the properties of transport current at the bottom of the bias triangles in different source and drain biases,observing the phenomenon of Pauli spin blockade.I speculate that the lift mechanism of Pauli spin blockade is spin-orbit coupling in germanium.Then I measure the anisotropy spectra of leakage current and demonstrate the corresponding lift mechanism of spin blockade qualitatively.My work lays a foundation for the manipulation and readout of spin qubits with high fidelity in germanium.4.I frabricate and measure the properties of the strain-germanium heterostructure one-dimensional four-quantum dot device,measuring the transport current.My experiment results prove the good tunablity between adjacent quantum dots and scalabity.Then,I use the method of virtual gate to realize the efficient individual manipulation of quantum dots,which provides ideas for the subsequent large-scale quantum dots manipulation.Innovations in this paper include:1.Fabrication and characterization of the transport properties of well tunabled double quantum dot devices in strained Germanium heterostructure for the first time.Study the transport properties of the devices,proving the good tunability of tunnel coupling between the double quantum dots.2.Observation of the phenomenon of Pauli spin blockade in double quantum dot devices in strained Germanium heterostructure for the first time,stduying the corrsponding lift mechanism.it proves that the strength of spin orbital coupling in germanium is quite strong,which lays a solid foundation for the manipulation and readout of spin qubits in germanium.3.Fabrication and measurement of the transport properties of well tunabled quadruple quantum dot devices in strained Germanium heterostructure for the first time,proving the good tunability of tunnel coupling between the adjacent double quantum dots.Realization of the efficient individual manipulation of quantum dot with the method of virtual gate,providing an effective way for the manipulation of large-scale quantum dots. |
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